Preface

The solar wind is a continuous outward stream of energetic charged particles from the Sun's hot corona. The high temperature in the solar corona measures more than one million degrees causing ionization of the hydrogen and formation of a hot plasma of protons and electrons. The solar plasma is so hot that it breaks free of the Sun's gravitational force and blows away from the surface in all directions giving rise to the solar wind. The intensity of the solar wind changes constantly, and when it gets stronger, we see more brighter aurora on Earth. Terrestrial magnetic field is compressed by the solar wind and distorted into a comet-shaped cavity known as the magnetosphere. The magnetosphere protects the Earth as it deflects the solar wind streams, which would otherwise blow the atmosphere away. However, the energetic solar flares and coronal mass ejections during times of an active Sun can drastically affect the solar wind and space weather conditions, and, implicitly, the advanced space technology we have become so dependent upon in our everyday lives. Understanding the changing solar wind and its effects on Earth and our life is therefore one of the most challenging tasks facing space scientists today, and many space exploration missions focus on the solar wind and its interactions with Earth.

This book consists of a selection of original papers of the leading scientists in the fields of Space and Planetary Physics, Solar and Space Plasma Physics with important contribu- tions to the theory, modeling and experimental techniques of the solar wind exploration. All chapters of this book were invited with the aim of providing a comprehensive view of the current knowledge of the solar wind formation and elemental composition, the interplane- tary dynamical evolution and acceleration of the charged plasma particles, and the guiding magnetic field that connects to the magnetospheric field lines and adjusts the effects of the solar wind on Earth.

The book is divided into five distinct sections: an introductive description of the solar wind properties and laws associated with different phases of the solar activity, and four key research topics with significant advances in the last decades. In the second section, the interested reader can find an extended analysis of the solar wind matter and elemental composition as measured in-situ by different spacecraft missions or from traces in microme- teorites. The third section is devoted to the solar wind dynamics ranging from the large-scale perturbations in the heliosphere to the smallscale kinetic processes of the wave-particle en- ergy dissipation. Magnetic reconnection is closely related to wave turbulence, which can be an efficient mechanism to dissipate magnetic energy into kinetic energy in small-scale, lo- calised processes. The fourth section highlights the role of the interplanetary magnetic field, which is powered by the Sun and extends through the corona further out in the solar wind. In the last section, four chapters report on the progress made in describing the solar wind interaction with the Earth's magnetosphere, focusing on principal geophysical effects as well as the wave turbulence and the problem of plasma penetration into the magnetosphere. The pressure exerted by the solar wind on the terrestrial magnetosphere has inspired a new and ambitious concept of propulsion for the so-called magnetic solar wind sails, which are the subject of the last chapter of our book.

It is necessary to point out that this book is not a monograph as it does not cover all aspects of the topic. Its purpose is to provide the means for interested readers to become familiar with the basic concepts as well as the recent progress in developing the observational techniques and theoretical models of the solar wind. I also am convinced that most of the research scientists actively working in this field will find in this book many new and interesting ideas.

> **Marian Lazar**  Institute for Theoretical Physics, Institute IV: Space and Astrophysics, Ruhr-University, Bochum Germany
